Gas turbine engines in aircraft such as helicopters which may be required to perform in dusty or sandy conditions are usually fitted with devices designed to separate incoming particles from an airstream entering the engine. Such particles entering the engine can cause damage to engine parts, thus creating potentially hazardous situations, and in any event may reduce the efficiency of the engine.
One type of device commonly used to separate particles from an incoming airstream is a vortex tube. An example of a known vortex tube is illustrated schematically in FIGS. 1 and 2 of the accompanying drawings, wherein FIG. 1 shows a longitudinal section through a vortex tube 10 and FIG. 2 shows a cross-section through the tube of FIG. 1 taken at line II--II.
The vortex tube 10 of FIGS. 1 and 2 operates by providing stationary swirl vanes 12 within a cylindrical air intake 14. The swirl vanes 12 are designed to impart a swirling, or vorticular, motion to the incoming airstream of contaminated air (shown by shaded arrow 6). Centrifugal forces brought about by the swirling motion act on particles entrained in the airstream and impart radial motion to these particles whereby they travel radially outwards to a scavenge annulus 16 at a downstream end 18 of the vortex tube 10 as an exit airstream of dirty air (shown by black arrow 7). The particles are directed from the scavenge annulus 16 into the ambient airstream outside the vortex tube 10. The remainder of the airstream within the vortex tube 10, now substantially free of particles (as indicated by white arrow 8), is directed to an axial exit tube 20 leading to the engine.
For efficient operation of a vortex tube the ratio of the cross-section area of the exit tube 20 at line II--II to the cross-section area of the scavenge annulus 16 at line II--II should be about 9:1. However, this ratio is in practice reduced to considerably less than 9:1, perhaps to a ratio approaching 1:1, and this in turn leads to a tendency for the air flow to separate immediately upstream of the scavenge annulus 16 at region 22 within the vortex tube (FIG. 1). This separation of the flow causes high pressure loss and reduces separation efficiency.
It is an object of the present invention to reduce high pressure loss in the vicinity of the scavenge annulus or, alternatively, to increase air flow through the vortex tube at constant pressure loss.